In the class of hearing aids generally referred to as implantable hearing instruments, some or all of various hearing augmentation componentry is positioned subcutaneously on, within or proximate to a patient's skull, typically at locations proximate the mastoid process. In this regard, implantable hearing instruments may be generally divided into two sub-classes, namely semi-implantable and fully implantable. In a semi-implantable hearing instrument, one or more components such as a microphone, signal processor, and transmitter may be externally located to receive, process, and inductively transmit an audio signal to implanted components such as a transducer. In a fully-implantable hearing instrument, typically all of the components, e.g., the microphone, signal processor, and transducer, are located subcutaneously. In either arrangement, an implantable transducer is utilized to stimulate a component of the patient's auditory system (e.g., tympanic membrane, ossicles and/or cochlea).
By way of example, one type of implantable transducer includes an electromechanical transducer having a magnetic coil that drives a vibratory actuator. The actuator is positioned to interface with and stimulate the ossicular chain of the patient via physical engagement. (See e.g., U.S. Pat. No. 5,702,342). In this regard, one or more bones of the ossicular chain are made to mechanically vibrate causing stimulation of the cochlea through its natural input, the so-called oval window.
For a wearer of an implantable hearing instrument, the sound of a speaker's voice reaches his inner ear by at least three different pathways. One of them goes from the vocal chords through the vocal tract, the outer air, the external ear canal, and the middle ear and to the cochlea; this will be called the air conduction pathway. A second pathway includes the vocal chords, the bony structure of the head and the inner ear; this will be called the bone conduction pathway. In persons without hearing loss, the relative level of acoustic signals reaching the inner ear via these two pathways determines the particular sound quality of an individual's own voice. For persons wearing an implantable hearing instrument, a third pathway is added: sound emanating from the vocal chords passes through the bony structure of the head and reaches the implanted microphone of the implantable middle ear hearing system or fully implantable cochlear implant. The vibration reaches the microphone diaphragm and is amplified just like an external airborne sound would be amplified. Also, in systems employing a middle ear stimulation transducer, the system may produce feedback by picking up and amplifying vibration caused by the stimulation transducer. As such, the bone vibration undesirably limits the maximum achievable gain of the implantable hearing instrument.
As may be appreciated, implantable hearing instruments that utilize an implanted microphone require that the microphone be positioned at a location that facilitates the receipt of acoustic signals. For such purposes, such implantable microphones are most typically positioned in a surgical procedure between a patient's skull and skin, at a location rearward and upward of a patient's ear (e.g., in the mastoid region). Because the diaphragm of an implantable microphone is covered by skin and this skin represents an additional mass loading of the diaphragm, vibration sensitivity of implanted microphones tends to be significantly higher than that of microphones in air. In order to achieve a nearly natural quality of the implant wearer's voice and increase achievable gain, the vibration sensitivity of the implanted microphone has to be reduced compared to its acoustic sensitivity. The aim of the present invention is to design an implantable microphone that achieves these goals.